docs: def _to_use(self):
          """
          Returns a tensor in the USE reference frame
          """
          if self.ref_frame is 'NED':
              # Rotate
              return utils.ned_to_use(self.tensor), \
                  utils.ned_to_use(self.tensor_sigma)
          elif self.ref_frame is 'USE':
              # Already USE
              return self.tensor, self.tensor_sigma
          else:
              raise ValueError('Reference frame %s not recognised - cannot '
                               'transform to USE!' % self.ref_frame)